Light probes are currently used to measure vibration in jet engine compressor blades, as a replacement or supplement to, the use of more costly strain gauges. Light probes also have the advantage of measuring vibration in all of the blades on a wheel which realistically cannot be accomplished with strain gauges. On the other hand, the ability to locate the light probes accurately and securely is critical to acquiring reliable data. Typical installations require many probes which make the installation cost a real concern. Various mounting techniques for light probes have been developed, but these have not been satisfactory in all respects.
In one prior installation, a 0.94 inch diameter probe was inserted in a 0.105 inch diameter hole in a compressor casing wall. The procedure did not provide for accurate placement, and vibration of the probe itself within the hole created the potential for "white noise" in the results. In addition, air pressure against the probe could result in movement of the probe outside of its measurement range.
Other mounting techniques include the use of threaded inserts, but these require additional machining and multi-piece assemblies which create the potential for debris entering the flowpath.
The present invention seeks to alleviate the installation problems experienced in the past by providing an inexpensive, pressure tight, and reliable assembly and related installation process for light probes used particularly for measuring displacement of land based gas turbine compressor blades.
In accordance with an exemplary embodiment of the invention, a light probe is fitted into a tube, with the forward end of the tube swaged to retain the light probe within a few thousandths of an inch from the tube end. The light probe and surrounding tube are installed in the compressor casing wall by means of a conventional swage lock pressure fitting. During installation, the forward end of the tubing surrounding the light probe is bottomed against the swaged end of the surrounding tube, with the forward edge of the tube flush with the casing inner wall, so that the probe is maintained at a predetermined distance from the inner casing wall, and hence the compressor blades. Tightening of the swage lock cap forms a pressure sealed assembly.
Accordingly, in its broader aspects, the present invention relates to a light probe assembly for mounting in a casing wall comprising a swage lock fitting sized to receive an outer tube; a light probe extending through the fitting and received within the outer tube; wherein the fitting is adapted for attachment to the casing wall, and wherein a remote end of the light probe is seated on an internal shoulder at a forwardmost end of the outer tube.
In another aspect, the invention relates to a method of installing a light probe in a casing wall comprising a) attaching an elongated tube having a crimped remote end to a fitting body; b) attaching the fitting to the casing wall; c) attaching a separable cap portion of the fitting onto the light probe; and d) inserting the light probe and cap portion into the fitting body with a remote end of the light probe seated on the crimped remote end of the tube.
Other features and advantages of the invention will become apparent from the detailed description which follows.